1. Field of the invention
The present invention relates to a method and composition for providing the surface of aluminum and its alloys with a coating to protect against corrosion or to improve adhesion of paint. In particular, the invention relates to a composition and method that use cerium salts to provide an improved coating on aluminum and aluminum alloys.
2. Description of the Background Art
Aluminum and aluminum alloys are frequently used to form structures, such as for aircraft, in which corrosion resistance is required or in which good paint adhesion is required. Aluminum has a natural oxide film which protects it from many corrosive influences. This natural oxide is, however, not sufficiently resistant to such highly corrosive environments as saltwater, nor is it a good base for paints. Improved films, which are both more corrosion resistant and suitable as a base for paints can generally be formed on the surface of aluminum either by anodizing or by chromate conversion. During the anodizing process, aluminum oxide is formed on the aluminum surface, and provides a very corrosion resistant surface which can be dyed or painted. However, anodizing has the disadvantages of high electric resistance, higher cost, longer processing time, and the need to make direct electrical contact with the part. This latter requirement complicates processing considerably.
Chromate conversion coatings are formed by dipping the aluminum part in chromatic acid, to provide a coating comprising chromium oxide(s) mixed with aluminum oxide. Chromate conversion coatings are corrosion resistant, provide a suitable base for paint, can be rapidly applied, self-heal when scratched, and are very cheap. Furthermore, chromate coatings are reasonably conductive and can be used in sealing surfaces for electromagnetic interference gaskets. The conductive characteristics provided by chromate conversion coating are not characteristic of anodized coatings nor of most protective coatings. Unfortunately, the hexavalent chrome used in producing these cheap, reliable and useful coatings poses serious health hazards as well as significant disposal problems. Dermatitis and skin cancer have been associated with the mere handling of chromated aluminum parts. Severe damage to mucous membranes and skin lesions called "chrome sores" occur from exposure to the ever-present chrome mist in plating shops. Such health hazards to humans represent a major problem in the use of chrome for protecting aluminum. Thus, it would be desirable to replace the chromating process entirely.
A recently developed process which eliminates the use of chromium involves coating aluminum surfaces with a film of aluminum oxhydroxide (pseudo boehmite), as disclosed in U.S. Pat. No. 4,711,667 for "Corrosion Resistant Aluminum Coating". This process yields a coating which is not as conductive as a chromate conversion coating, but is not, however, an insulator. In addition, its corrosion resistance is not as good as that produced by chromate conversion. The details of this known process are discussed in Example 1 herein.
In another known method, aluminum has been treated with cerium chloride, CeCl.sup.3, to form a mixed cerium oxide/cerium hydroxide film on the surface, as described, for example, by Hinton, et al., in the publication "Cerium Conversion Coatings For The Corrosion Protection of Aluminum," Materials Forum, Vol. 9, No. 3, pages 162-173 (1986). In this process, a coating of cerium oxide/hydroxide is precipitated on the aluminum surface and provides a relatively high degree of corrosion resistance. Unfortunately, this process is slow, taking almost 200 hours. The speed of the process can be improved so that the coverage occurs in 2 to 3 minutes by cathodically polarizing the coupon. However, this leads to a less durable coating, and the process is inconvenient because it requires the use of electrodes.
Thus, it would be desirable to provide a chromium-free process for providing aluminum and aluminum alloys with a protective coating which is rapid and does not involve the use of electrodes.